A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Typically, a pattern that is to be imaged on a layer of the substrate should be aligned with one or more patterns that have been imaged in a respective preceding patterning step. To this end, optical alignment methods are known that employ alignment marks on the substrate to obtain position and orientation references.
Alignment marks consist of gratings that have periodicity larger than the wavelength of an alignment illumination beam. During an alignment procedure the alignment illumination beam impinges on the grating, and from the diffracted light as generated by the grating an alignment sensor can obtain the information on the position and orientation of the substrate.
For proper processing the constituent parts of the alignment mark that typically consist of the same material as (parts of) device features, should generally have dimensions similar to dimensions of the device features that are manufactured by the lithographical processing to avoid size-induced deviations during processing of integrated circuits, due to, for example, a micro-loading effect during a reactive ion etching process which may occur at device structures in the vicinity of a large marker area or due to size dependency of chemical-mechanical polishing (CMP) of structures. To comply with processing conditions and state-of-the-art design rules sub-segmented marks are being used. The sub-segmented marks that are being used are marks that consist of perpendicular sub-alignment wavelength lines, e.g., the polar mark, and marks that comprise contact holes.
In double patterning technology (DPT) extreme dipolar illumination settings may be used. It is observed that due to the illumination conditions, alignment marks, and more in particular polar marks and marks that consist of contact holes, may not be imaged properly. This results in poorly defined alignment marks or even in a failure to create alignment marks. Use of poorly defined marks in optical alignment may lead to loss of signal. If the poor printability results in an alignment mark with an unintentional asymmetry, such an alignment mark may cause an alignment position shift.